A beam splitter is a commonly used element in optical apparatus. A beam splitter splits an incident beam into two output beams which diverge at a fixed angle. Other commonly used beam splitting prisms split an incident beam into two substantially parallel output beams such as described in the book of M. Francon, and S. Mallick, "Polarization Interferometers", Wiley-Interscience, 1971, pp. 25. However, these prisms have the disadvantage of the angle of separation between the two exit beams of the prism is fixed. Continuously variable beam splitters are described in the article of R. Drougard, and J. Wilczynski, "New Polarization Interferometer For Fourier Analysis", J. Opt. Soc. Am. 55, 1638 (1965) and in the book of M. Francon et al. at pages 40-42. However, the Drougard beam splitter requires complex matched counter rotation of two
Wollaston prisms relative to a precisely-aligned, precision half-wave plate to avoid the generation of four exit beams from the variable beam splitter. This approach results in a variable beam splitter that is difficult to operate and that requires several expensive optical components.
Optical apparatus such as the optical apparatus described in U.S. Pat. No. 4,758,092 describes an apparatus and technique for probing dynamic sheet charged density variations in an integrated semiconductor device. This apparatus uses a beam splitting prism in a phase-contrast interferometer which optically detects the electrical signals in a functioning electronic circuit. The operation of such an apparatus can be substantially enhanced by a simple to use continuously variable beam splitter. The teaching of U.S. Pat. No. 4,758,092 is incorporated herein by reference.